One well-known form of amplifier is the feed-forward amplifier. In order to achieve linearity in a feed-forward amplifier, careful control of the amplifier circuitry is required. In particular, in feed-forward amplifiers two or more gain and phase adjusters are often employed and the taps of each of these adjusters are carefully steered to achieve linearity through the amplifier.
Within the art of feed-forward amplifiers, it is known to use detector-controller circuits, one for each gain-and-phase adjuster. Each detector-controller circuit is operable to steer the taps of its respective gain-and-phase adjuster in the feed-forward amplifier so that the main amplifier and correctional amplifier can properly cooperate in order to reduce error introduced by the main amplifier.
Detector-controller circuits used in feed-forward amplifiers use a number of different techniques to control the phase and gain adjustments in signal and intermodulation cancellation loops. Typically, the signal cancellation loop is nulled by measuring the total power at the cancellation node and adaptively minimizing it. This makes sense because the undistorted input signal is being removed (cancelled) from the composite signal. The total power will be minimized when this cancellation is a maximum. The intermodulation cancellation loop can be nulled using a number of techniques including pilot tones, intermodulation detectors, vector signal analysis, and many others.
A first order or textbook analysis of a feed-forward amplifier would suggest that only undesired distortion products should be amplified by the correction amplifier. In practice, this turns out to be neither possible nor desirable. There will always be some residue of the original undistorted signal in the correction amplifier signal. This is because (1) signal cancellation at the summing node is not perfect, and (2) the main amplifier in the signal cancellation loop will characteristically be driven into non-linearity on signal peaks, resulting in loop imbalance. Perfect cancellation could only be achieved in a perfectly linear system.
Conventional wisdom has been that the signal cancellation loop should be adjusted to yield the best overall cancellation possible, given the dynamic range of the signal used. Again, this is characteristically done by minimizing the total power at the summing node, although it may be done by other techniques such as pilot tone nulling.
The correction amplifier is typically a rather large amplifier that is sized to be able to handle the peak power demands placed upon it by the correction signal. Herein lies one of the oft-mentioned disadvantages of feed-forward amplifiers versus other approaches; the wasted power and cost of the correction amplifier.
Most well-designed feed-forward amplifiers use gain and phase adjustments in series and in front of the main amplifier in the signal cancellation loop. The most important reason for doing this is to maintain constant gain of the feed-forward amplifier in the face of drift in the main amplifier caused by factors such as temperature variation and component aging. Because the loss in the delayed path in the signal cancellation loop can generally be counted upon not to vary with temperature, placing the gain adjustment in series with the amplifier, and maintaining constant signal cancellation, will result in constant gain. This is very useful from a systems point of view, although it is not, strictly speaking, necessary.